Field
Embodiments described herein generally relate to methods and systems for fluidized catalytic cracking. More particularly, such embodiments relate to methods and systems for fluidized catalytic cracking using modified catalysts.
Description of the Related Art
Fluid Catalytic Cracking (FCC) is a technology used in refineries to produce transportation fuels such as gasoline and distillates and other liquid and/or gaseous hydrocarbon products from higher molecular weight feedstocks. The FCC process uses a reactor called a riser, essentially a pipe, in which a hydrocarbon is contacted with fluidized catalyst particles to effect the conversion of the hydrocarbon to more valuable products. For example, the FCC unit can convert gas oil by “cracking” the gas oil molecules into smaller molecules. The resulting hydrocarbon product and catalyst mixture both flow through the reactor, hence the term fluid catalytic cracking.
For the catalytic cracking of traditional refinery gas oil feedstocks such as atmospheric and vacuum gas oils, catalysts, such as catalysts containing Y or USY zeolites, are used with pore diameters large enough to allow diffusion of the feed and products into and out of the catalyst.
For heavy FCC feedstocks, such as atmospheric residue or vacuum residue containing feedstocks, the pore diameters of these zeolites may be too small to allow diffusion of the largest feed molecules into the active sites within the zeolites. Therefore, for heavy FCC feedstocks, catalytically active non-zeolitic materials, such as various forms of amorphous alumina and powdered clays with even larger pore sizes, are often included in the catalysts to allow the largest molecules to crack to some extent on non-zeolite surfaces so that the fragments can then diffuse into the smaller zeolite pores for further cracking. These catalysts are commonly referred to as catalysts with active matrices. The product selectivity from cracking on the non-zeolite catalytic materials are generally not as favorable for the production of the most desired FCC products and produce higher yields of coke and light gases compared to the zeolite catalyzed cracking. Therefore, the catalysts employed in processing of very heavy FCC feedstocks are often formulated with the intent to provide a balance between the zeolite and active matrix contributions to the catalytic surface areas.
For the catalytic cracking of light feeds such as liquefied petroleum gas (LPG) or light naphtha, in the absence of heavier feed components, smaller pore diameter zeolites, such as ZSM-5, sometimes referred to as medium pore zeolites, are typically employed. These zeolites have greater activity for cracking the light feeds than do the larger pore size zeolites, and limitations to diffusion of feedstock molecules or product molecules larger than those present in the feedstock into the zeolite has not been considered a relevant issue.
The feedstock entering the riser is heated to the desired cracking temperature because the cracking reactions are endothermic. During the cracking of heavy feeds coke is formed within the catalyst. The coke deposits are typically burned with an oxygen source such as air in a regenerator. Burning the coke is an exothermic process that can supply the heat needed for the cracking process. In a heat balanced operation, typical of most FCC operations, the quantity of coke formed on the catalyst is significant enough that no external heat source or fuel is needed to supplement the heat from coke combustion.
On the other hand, unlike heavy feeds, light feeds do not deposit enough coke on the catalyst in the reactor to support the proper heat balance of the FCC unit. In these cases, an external source of fuel or other heat input can be required to keep the FCC unit in heat balance. Adding external heating sources or fuel directly to the regenerator of the FCC unit can increase the capital cost, operational expenditures, and/or complexity of the process. At the same time, because of the extremely high reaction temperatures employed in cracking light feedstocks, coke can be aggressively deposited or formed in the FCC reactor hardware. This coke can damage refractory in the FCC hardware and/or plug the internals of the reactor. The extent of this coking can be severe enough to require shutdown to remove the accumulated coke and replace or repair the damaged refractory.
The high temperatures employed in catalytic cracking of light feedstocks can also produce a gasoline product with unacceptably high concentrations of dienes, other olefins, and/or other reactive species that can cause the gasoline product to fail compliance with applicable motor fuel quality specifications such as potential gum formation as determined by ASTM D-525 and ASTM D-873.
There is a need, therefore, for more improved methods and systems for cracking light hydrocarbon feeds with a reduced need for external heating, a reduced diene and/or other olefin concentration in the hydrocarbon products, and/or a reduced propensity to foment coke formation and/or coke deposits within the system.